What is a key indicator of coolant level in a vehicle?

The key indicator of coolant level in a vehicle is the sight gauge, which provides a visual reference of the fluid level in relation to the minimum and maximum marks. This allows for quick verification of whether the coolant is within the acceptable range for safe operation. Maintaining the coolant level between these two indicators is vital, as insufficient coolant can lead to engine overheating, while excessive coolant can cause issues, too.

The other options relate to coolant levels but do not accurately represent the best practice for checking them. For instance, having the coolant above the maximum line may indicate overfilling, which isn't advisable. Rather than relying on the presence of bubbles in the tank, which can suggest an air leak or overheating issue, checking the sight gauge provides a straightforward method for ensuring proper coolant levels. Lastly, having the gauge be empty is a sign of a problem rather than an acceptable state, which can lead to engine failure if not addressed promptly.